1. Field of the Invention
The present invention relates to an image heating device that heats an image on a sheet.
2. Description of the Related Art
An image forming apparatus, such as a printer, a copying machine, a facsimile machine, or a multi-functional apparatus of these machines, which includes a fixing device (an image heating device) of a belt heating type using a fixing belt (an endless belt) has hitherto been put into practical use. In such a fixing device, a toner image, which is formed and born on a sheet (a recording material) by an image forming method such as an electrophotographic process or an electrostatic recording process, is fixed on a surface of the recording material by heating.
Since such a fixing device of the belt heating type uses a thin fixing belt having low heat capacity and high thermal responsiveness, the temperature of the fixing belt can reach a fixing temperature in a short time from power-on. This greatly contributes to power saving of the image forming apparatus.
However, the thin fixing belt may be broken owing to deformation or a flaw caused by any external force. If the fixing belt is broken, not only an image defect may be caused, but also a broken part may contact and break other components. Therefore, if the fixing belt is broken, it is preferable to immediately understand the fact and to prohibit a fixing process (an image heating process). A technique for that purpose is proposed in Japanese Patent Laid-Open No. 2002-287542.
Specifically, in a fixing device described in Japanese Patent Laid-Open No. 2002-287542, a belt mark is put on a fixing belt, and an optical sensor is disposed on a side opposed thereto. With this structure, it is determined that the fixing belt is broken when the optical sensor does not detect the belt mark for a fixed time.
However, in the fixing device described in Japanese Patent Laid-Open No. 2002-287542, if a flaw is made on the belt mark or a foreign substance adheres to the belt mark, the amount of light (amount of reflected light) received by the optical sensor decreases and becomes unstable. Hence, a breakage of the fixing belt may be detected erroneously.